The invention relates to a polyamide filament comprising at least 80% by weight of nylon 66, a process for producing it and its use.
Nylon 66 has excellent thermal-mechanical properties It is therefore highly suitable, inter alia, for producing filaments via melt spinning However, as a characteristic disadvantage of this processing of nylon 66 polymer it must be borne in mind that a phenomenon known as low luster or LL sensitivity is observed It is due to spherulite inhomogeneities which, in the course of the spinning or spin-drawing process, are formed in the cooling filament underneath the spinnerette The consequences thereof are in some instances strongly adverse effects in particular on the course of spinning and drawing and on the mechanical properties. The LL phenomenon arises in particular at high filament deniers, which cool down in the quenching chamber more slowly than thin filaments
LL can be triggered artificially by lowering the melt temperature in the region of the point of exit from the spinnerette (so-called temperature LL). By this method it is possible to examine various measures, for example chemical modification to the nylon 66 matrix, in respect of their suitability for reducing LL sensitivity or spherulite formation.
The prevention of spherulite formation has long been of intense interest For instance, US-A-4 919 874 discloses a process for producing nylon filaments by mixing nylon 66 with nylon 6 and copolymerizing hexamethyleneadipamide with .epsilon.-caprolactam. Spherulite formation is encouraged by adding a water-soluble inorganic calcium salt. The purpose is not to prevent but to produce a large number of very small spherulites in very homogeneous distribution. In addition to the fact that such nylon 66 filaments are still full of spherulites, a further disadvantage is the lower melting point which has to be accepted as a result of the addition of .epsilon.-caprolactam. Nor can the sole addition of caprolactam solve the LL problem, since the influence of the addition of .epsilon.caprolactam within a co-content range of 1-3% by weight on LL sensitivity is low.
Increasing the caprolactam content to above about 3% has the disadvantage that the melting and softening point is lowered to such an extent that further processibility, specifically texturability, is impaired. Besides, high caprolactam concentrations also have an extremely adverse effect on the thermo-mechanical properties, for example on the crimp modulus.
An addition of less than 1.0% by weight of caprolactam compared with nylon 66 containing no addition does not show any significant reduction in LL proneness.
The numerous attempts undertaken for the purpose of preventing spherulite formation and thus the LL effect also include increasing the spinnerette temperature, shortening the residence time of the melt and frequent spinnerette replacement. However, none of the numerous measures has had a satisfactory result.